In the Figures, the same element in different figures will be indicated by the same reference numeral. Closely related elements will be designated by the same reference numeral, but with differing numbers of primes on that numeral.
This invention relates in general to hub-to-shaft couplings and relates more particularly to a clamping bush that reversibly locks a hub to a shaft by means of a reversible compression fit. By a "hub" is meant any element that has a cylindrical bore therethrough. Typically, the hub is part of a disc, wheel, lever arm or similar component and functions to attach such component to the shaft. By a "shaft" is meant any cylindrical bar. Usually, but not always, the shaft will have a circular cross-section in a plane normal to the axis of the shaft and the hub will be cylindrically balanced to rotate about the center of its cylindrical bore without producing a significant amount of wobble. By a "bushing" is meant a removable lining for a shaft.
In general, hub-to-shaft couplings are used to transfer torque between the shaft and the hub. In an early hub-to-shaft coupling, a square shaft was fitted into a substantially equal size square opening in a hub. In a subsequent hub-to-shaft coupling, a circular shaft was fitted into a substantially equal size circular hole in a hub. A flat or tapered key is forced between the shaft and hub to firmly wedge between the shaft and the hub and enable torque to be transmitted between the shaft and hub. A common hub-to-shaft coupling utilized today employs a key in a keyway formed in the shaft and hub to prevent rotation of the hub relative to the shaft. Such a key/keyway system can involve a cylindrical shaft having a rectangular cross-section fitted within a rectangular keyway that extends into both the shaft and hub. A splined shaft and associated splined bore in the hub as used in the automotive gear and in the machine tool industry is another example of the key/keyway type hub-to-shaft couplings.
Despite the decade long use of such couplings, a key/keyway type hub-to-shaft coupling has many disadvantages that has led to many attempts to replace it by a better system. The main disadvantage of the key/keyway system is the notch sensitivity of both the hub and shaft. To avoid this sensitivity, both components are often overdimensioned. However, this results in shafts that are larger than necessary for transmitting the torque, resulting in excessive weight of parts, including components attached to the hub and shaft, such as brake drums, brake discs, flywheels, gear wheels and couplings.
In key/keyway type couplings, the machinery user runs the risk of sudden machine failure whenever peak torques occur that exceed the design limits. Such design limit can be exceeded as the result of shocks, such as can be delivered by a drive train. Also, under repeated peak load operation, clearance between the sides of the key and the keyway leads to irregular dynamic operation such as problems with speed control and can result in damage to the hub-to-shaft coupling and the entire drive system.
Another hub-to-shaft coupling called the K-profile is similar to the square shaft embodiment mentioned above, except that the cross-section of the hub is triangular. Unfortunately, this coupling is expensive and requires the close sliding fit that is needed in the splined hub-to-shaft coupling discussed above.
Shrinkage fit type hub-to-shaft couplings avoid many of the problems discussed above. In such embodiments, the outer member can be heated temporarily to let it slip over the inner member or the inner member can be cooled to let the outer member slip over it. After both members reach ambient conditions, there will be a tight fit between these two members. This approach avoids the excessive material requirements and notch sensitivity discussed above for the key/keyway type hub-to-shaft couplings. However, shrinkage fit couplings are hard to unlock and can be destroyed during disassembly. Implementing the shrinkage fit requires great skill. If the wrong temperature is utilized, the shaft and hub can get stuck before being properly aligned. Even when this process is carefully performed by skilled worker, some components can be destroyed.
The hydraulic fit system is easier to handle, but requires the expensive machining of oil ducts into the components. These oil ducts include a threaded portion that is adapted to receive a threaded pressure hose to force oil between the shaft and hub. This hub is coupled to the shaft by a shrink fit method as discussed above. The hub can be removed from the shaft by forcing oil into the ducts to expand the hub away from the shaft, enabling it to be slipped off of the shaft. Unless special tools are used, heating of parts is still necessary. However, dismantling of the coupling is at least possible without major problems.
In another type of shrinkage fit system, single or double wedge-type rings are utilized either to clamp the hub to the shaft from the outside of the hub, to clamp the hub to the shaft from the inside of the bore of the hub or to clamp the hub to the shaft with a conical sleeve that is hydraulically pressed axially into the gap between the shaft and hub. All of these systems require a large number of bolts, are complicated, are expensive and require skill for proper (especially concentric) positioning of the components. The tapered bush coupling must also be of the key/keyway type coupling if it is to be used in the upper torque range of its design specifications.
In the automotive industry, it is also known to use hydraulic expansion of components such as the cam and cam shaft. However, such an approach requires a shaft having an internal bore and is an expensive, high precision process. Also, the shrinkage is irreversible so that these parts cannot be easily disassembled.
In the double-wall bushing type hub-to-shaft coupling, one end of the bush is welded shut and the other end remains open to receive an annular piston for compressing plastic fill between the walls. This compression expands the bush in both radial directions so that it presses against both the shaft and the hub. Annular piston bolts are required to maintain this pressure within the bushing. However, the pressure within this bush can decrease with time, thereby losing the clamping effectiveness. This type of clamp is relatively bulky and therefore requires more material and produces more inertial loading of the shaft than is desirable.
All of the clamping type systems discussed above that use conical elements are relatively large. They require a large amount of space for fitting and dismantling as well as to accommodate the external rings utilized in some embodiments to clamp the hub to the shaft.
An expanding mandrel can be used to clamp a hub to a shaft, but it requires an inner bore to house the central spindle and the cones that are threaded internally to expand the slotted outer housing of the mandrel and it is typically too long to be used effectively as a hub-to-shaft coupling.